The invention belongs to the field of electrical switches and particularly to a reversing switch for large three phase induction motors.
Bulk materials handling conveyors, such as those used in underground coal mines, are examples of electrically driven equipment which commonly requires reversing facilities. Electrical power equipment used near the working face in coal mines must be "explosion proof" to prevent electrical sparks or arcs from igniting any explosive mixture of air and methane or coal dust which may be present. Hence any electrical reversing switch intended for use on coal face equipment must be in an explosion proof enclosure. Reversing switch mechanisms previously available have not been easily or inexpensively adaptable for use within explosion proof enclosures.
In certain applications, such as the electric drive and controls for a coal plow operating along the face in a longwall mining system, the motor drive is operated forward and then reverse in a cyclic manner. The motor control equipment includes separate forward and reverse contactors which are operated alternately. That arrangement while effective for such special purposes is objectionably bulky and expensive for many other motor reversing applications such as conveyors which run in one direction to move coal out of the mine but are reversed from time to time to move supplies and personnel into the mine.
One reversing switch previously available for this purpose was disclosed in applicant's U.S. application Ser. No. 06/662,493 filed Oct. 18, 1984 on "REVERSING SWITCH", now U.S. Pat. No. 4,563,549.
Applicant's above-described prior switch was positively closed by cam mechanism on the handle which forced the contacts together. However, the contacts were spring-urged apart and could not be forced open by swinging the handle. While the switch operated satisfactorily, it has since been recognized that there would be an advantage in positively opening the contacts as well as positively closing them with the handle.